Method of manufacturing a fiber-optical element

ABSTRACT

IN THE METHOD OF MANUFACTURING A FIBER-OPTICAL ELEMENT IN WHICH A ROD-SHAPED GLASS BODY HAVING A HIGH REFRACTIVE INDEX AND CONTAINING A SMALL QUANTITY OF MONOVALENT IONS SURROUNDED BY A GLASS SHEATH OF LOW REFRACTIVE INDEX AFTER BEING DRAWN INTO GLASS FIBERS WHICH ARE ASSEMBLED INTO A BODY WHICH IS AGAIN DRAWN TO REDUCE ITS SIZE AND THE FIBERS FUSED INTO A COMPACT STACK WITH MINIMUM DEFORMATION, THE MONOVALENT IONS ARE EXCHANGED FOR OTHER IONS WHICH INCREASE THE LIGHT ABSORPTION BY THE SHEATH.

SEARCH ROOM June 971 H. T. P. F. LAKEMAN 3,582,297

SUBSTTUTE FOR MISSING XRETHQD OF MANUFACTURING A FIBEROPTICAL ELEMENTFiled Aug. 28, 1967 FIG.1

FIG.2

INVENTOR. HENRICUS TH,P. F. LAKEMAN United States Patent Othce 3,582,297Patented June 1, 1971 6 3 7 Int. Cl. C030 /00, 23/20 U.S. Cl. 65-4 4Claims ABSTRACT OF THE DISCLOSURE In the method of manufacturing afiber-optical element in which Fad-shaped glass body haVWctive index andcontaining a small quantity of monovalent ions surrounded by a glasssheath of low refractive index after being drawn into glass fibers whichare assembled into a body which is again drawn to reduce its size andthe fibers fused into a compact stack with minimum deformation, themonovalent ions are exchanged for other ions which increase the lightabsorption by the sheath.

The invention relates to a method of manufacturing a fiber-opticalelement and to an element obtained by such a method.

In recent years, fiber-optical elements which consist of a bundle ofglass fibers of very small diameter serving to transmit light arefrequently used in cases in which even images of very small brightnessare transmitted without definition losses due to dispersion. They areused, for example, in image intensifiers and in television camera tubes.

The effect of such a fiber is due to the fact that a light beam which isincident on one end face of the fiber is reflected completely from theside walls of the fiber and thus remains substantially entirely insidethe fiber and reaches the other end with practically the same intensity.In order to achieve this, such a fiber comprises a cylindrical core of amaterial having a high refractive index (n concentrically surrounded bya sheath of glass having a low refractive index (n The term cylindricalis employed herein to signify any form of constant closed crosssection,hence not only a circular but also a rectangular or polygonal form.

It is known to manufacture a fiber-optical element by drawing arod-shaped body comprising a core of glass having a high refractiveindex and a concentrical sheath of glass having a low refractive indexto fibers having crosssections approximately corresponding to that ofthe rodshaped body, whereupon the so obtained fibers are joined to forma bundle and drawn again, which latter two operations may then berepeated. The resulting bundle is then fused together to a compact stackwith a minimum of deformation and the occurrence of foreign occlusions,especially of gas bubbles between the fibers is prevented.

As already stated, losses of definition due to dispersed rays should beavoided. For this purpose, first, the critical angle i.e., the anglebetween the light beam incident on the fiber-optical element and thenormal to the end face of the fiber, should be at a maximum. All thelight beams incident on the end face of the fiber at an angle which issmaller than this critical angle, remain inside the same fiber due tocomplete reflection. The relation between this critical angle 0, therefractive indices of the two kinds of glass and the refractive index ofthe ambience (n is:

By suitable choice of the refractive indices of the two kinds of glass,this critical angle 0 may be approximately when the fiber-opticalelement is arranged freely, i.e., in an ambience of air. This means thatall light, even the light inicident at an extremely small angle, once itenters into the fiber, remains in it due to complete reflection.

Glass are available for the sheath of the fiber which have a refractiveindex (21;) of approximately 1.50. The composition of these glasses inmol. percent lies within the following limits:

(i002 35-62%, preferably 4055% 13:10.... l030%, preferably l52l%......

........ 0 30%, preferably 3-13% 01527 preferably 340%...

In all at least 10%.

With a combination of one of the latter glasses for the core and one ofthe former glasses for the sheath, the

attainable value for /n -n lies between approximately 1 and well over 1.

The main source of a dispersed light is light incident on an end face ofthe fiber-optical element the glass of the sheath at an angle differingfrom 0. The glass of the sheath must naturally have a limited minimumthickness, since otherwise cross-talk" may occur between the fibers.This minimum value is of the order of a few tenths of a micron. Thismeans that in a cross-section of the fiberoptical element, the surfacearea of the sheath occupies 30 to 40% of the overall surface area. Whencompared with this source, other incidental errors producing scatteredlight, such as faults at the interface and gas bubbles or crystals inthe glass of the core of the fiber have only slight influence. Moreover,the presence of a zone having a refractive index (n which largelyexceeds 1, such as an immersion or a layer of luminescent material witha binder applied to the end face of a fiber-optical element, should betaken into account. The critical angle 6 is then considerably reduced,which already appears from the said equation n sin 0= /n n Variousattempts have already been made to absorb the scattered light.

For example, the fiber or a bundle of fibers may be enveloped with acolored glass or enamel. This has the disadvantage that the fillingfactor of the fiber-optical element, which corresponds in across-section of the element to the fraction of the overall surface ofthe cross-section of the element constituted by the sum of the surfacesof the cross-sections of the core of the fiber, decreases, because theglass sheath must have a thickness which is twice that required toprevent cross-talk. As a result, the light transmission of the elementis directly reduced.

As an alternative, the glass of the sheath may be colored, but this alsoinvolves an unduly large absorption.

The known method in which the surface of the element is etched with areagent which attacks the sheath of the fiber more strongly than thecore, whereupon a layer intpervious to light is applied which fills thecavities formed and is then ground and polished at the area of the core,is very circuitous. Moreover, this method results in an absorbing layerof small depth so that only part of the scat-.

tered radiation is eliminated.

When using the method according to the invention, a fiber-opticalelement is obtained in which definition losses owing to scatteredradiation are satisfactorily counteracted. Even scattered radiationresulting from defective areas in the core is eliminated by absorptionon the lower side of the fiber-optical element.

The method according to the invention is characterized in that in theglass of the sheath of the element obtained by the known method andcomprising a compact bundle of glass fibers constituted by a core ofglass having a high I refractive index and containing at the most a fewpercent of monovalent ions and a concentrical sheath of glass having alow refractive index, monovalent cations are exchanged for such othercations that the light absorption of this glass increases.

In a preferred embodiment of the method, alkali ions in the glass areexchanged for silver ions, since the diffusion velocity of the silverions in the glass is high. The glass may then contain electron donorssuch as AS203 or' Sb O so that the diffused silver ions are converted toblack-colored particles (conglomerates of metal atoms). In aparticularly suitable method, no reducing compounds are added to theglass, but the glass containing the silver ions is heated for some timein a reducing atmosphere.

The ions can be introduced by coating the surface of the fiber-opticalelement with a paste containing the relevant metal ions, by then heatingthe assembly and rinsing away the remaining paste after cooling, or thefiber-optical element may be immersed in a bath of a molten salt or saltmixture containing the relevant metal ions.

In another embodiment of the method according to the invention, thediffusion of the metal ions is accelerated by an electric field and alarger depth of penetration can then be obtained. For this purpose, onesurface of the fiberoptical element in a position at right angles to thelongitudinal direction of the fibers is coated with electricallyconducting paste containing the relevant metal ions, which surface thenacts as an anode and the opposite surface is coated with a graphitepaste and then acts as a cathode, the electric field being maintaineduntil the desired depth of penetration has been attained.

By treating the fiber-optical plate with silver ions, it I may becolored on both sides. A layer of to microns is sufficient toeffectively suppress the dispersed radiation. The fact that the glass ofthe sheath is not colored entirely from one side of the element to theother, but only for a fraction of approximately 1% of the thickness, isespecially favorable, since the internal absorption is thus considerablyreduced, so that the loss of light is much smaller than in theaforementioned known method in which the whole sheath is colored.

By the way, it should be noted that the method used in accordance withthe invention for exchanging alkali ions in the glass for ions whichincrease the light absorption, is particularly suitable to renderabsorbing the sides of a glass body which is provided on the window of acamera 7 tube of the Vidicon type for reducing halo effects, the

target plate of this tube, which is disposed on the inner side of thewindow, preferably consisting of lead monoxide as disclosed and claimedin French Pat. No. 1,389,809.

The invention will now be described with reference to the accompanyingdrawing, in whcih FIG. 1 is a perspective view (shown on a greatlyenlarged scale) of a part of a fiber-optical element obtained by themethod according to the invention, and FIG. 2 is a cross-section of apart of the element parallel to the longitudinal axis of the fibers.

First a glass tube was manufactured which has a wall thickness of 1 to1.5 mm., an outer diameter of 17.5 mm. and a length of 300 mm. and whichconsisted of glass having the composition:

Percent by weight sio 60.1

n n w w n A1 0 3.9 N8 0 13.4

This glass had a refractive index n =1.50 and a linear coefilcient ofexpansion of 70 l0 C. between and 300 C. A cylinder of the same lengthand of the following composition was fitted by grinding in this tube:

GeO 50.0 mol percent corresponding to 40.5% by weight BaO: 19.3 molpercent corresponding to 22.8% by weight TiO 7.7 mol percentcorresponding to 4.8% by Weight La O 7.0 mol percent corresponding to17.6% by weight ZrO 5.0 mol percent corresponding to 4.7% by weight Ta O1.0 mol percent corresponding to 3.4% by weight ZnO: 10.0 mol percentcorresponding to 6.2% by weight ampulla having a diameter of 25 mm. madeof borosilicate glass of the first-mentioned composition. The filledampulla was evacuated, sealed and heated for half an hour to one hour ata temperature of 680 to 700 C. Plateshaped fiber-optical elements werecut from the product obtained and immersed for 6 hours in a meltcontaining 20% by weight of AgNO and by weightof KNO; and heated at 350C. After cooling in air, the plates were carefully washed with distilledwater and then heated in hydrogen gas for half an hour at 500 C.

The sheathof the carefully cleaned plate was colored black on both sidesto a depth of l5,u and the glass of the core had remained perfectlytransparent. FIG. 1 shows in a perspective view a section through a partof such a fiber-optical element.

FIG. 2 is a cross-sectional view parallel to the axis of r the fibers,the core being denoted by 1 and the sheath colored black to a depth of15a by 2. Attention is drawn to the light beams 3 and 4 which obliquelytravel through the optical element and pass several times through thecolored sheath. Upon each passage, a large part is absorbed; thus, whenit has passed several times through a layer of sheath glass, the lightbeam is substantially completely extinguished. This dispersed light isthus completely eliminated, which becomes manifest in the considerableimprovement of the resolving power of a fiberoptical plate obtained bythe method according to the invention with respect to' a similaruncolored plate. The core has an irregularity 5 by which the light beam6, which hitherto remained inside the fiber due to complete reflection,is refracted so that it emerges from the fiber and laterally passesthrough the element. A similar light beam 7 is shown, which thus hasreached the lower side of the element. This scattered light beam is alsoabsorbed by the black-colored glass of the sheath.

While the invention has been described with reference to particularembodiments and applications thereof, other modifications will bereadily apparent to those skilled in the art without departing from thespirit and scope of the invention which is defined in the appendedclaims.

What is claimed is:

1. In the method of manufacturinga fiber-optical element comprising thesteps of drawing a rod-shaped body,

which comprises a core of glass having a high refractive index andcontaining at the most a few percent of monovalent ions and aconcentrical sheath of glass having a low refractive index into glassfibers having a structure approximately similar to that of therod-shaped body, concentrating said fibers to form an assembly, drawingsaid assembly to reduce the size thereof, fusing the resulting fiberstogether into a compact stack with a minimum of deformation, the step ofexchanging monovalent cations in the glass of the sheath of the elementfor other cations which increase the light absorption.

2. A method as claimed in claim 1 in which alkali-ions in the sheathglass are exchanged for silver ions under reducing conditions.

5 6 3. A method as claimed in claim 2 in which after dif- 3,218,22011/1965 Weber 6531XR fusion of silver ions, the fiber-optical element isheated 3,237,039 2/1966 Flyer 65LRDIGUX in a reducing atmosphere.3,379,558 4/1968 Upton 65 4X 4. A method as claimed in claim 2 in whichthe ex- 3,393,987 7/1968 Plumat 6530X change is accelerated by anelectric field acting in longi- 5 3,395,994 8/1968 Cuff 6330X tudinaldirection of the fibers. 3,436,142 4/ 1969 Siegmund et a1. 654X3,528,847 9/1970 Grego et a1. 6530X References Cited 3,495,963 2/1970Buckley et al. 65-30 3,486,808 12/1969 Hamblen 65-30X UNITED STATESPATENTS O FRANK W. MIGA, Primary Examiner 2,749,794 6/1956 OLeary 65-31X2,995,970 8/1961 Hick, Jr., et al. 65LRDIGUX 3,139,340 6/1964 Hays etal. 65-;LRDIGUX CL 3,195,2l9 7/1965 Woodcock et '81. 65-30, 43, 37;106-52 65--LRDIGUX 15

